100+ years ago, my house had a well in back that was housed in a large concrete enclosure below ground level. The entire enclosure is still there, with the old well head, but that well stopped producing many years ago.

I would like to put the concrete enclosure to modern use as a cistern to catch gutter drain off water. The problem is that the enclosure has a few horizontal cracks that go almost completely around and allow rainwater in when it rains. The cracks are not wide, but are about 3-4 feet below ground level.

Are there any particularly good hydraulic caulk or epoxy/cement products I would be able to use to fill there cracks?

i'm not a pro fan of apron stores,,, after 38yrs of const exp, you tend to wince when overhearing ' helpful ' conversations that will be useless in the end if those methods/materials are used,,, however, when i'm OUtside my areas of expertise, i have found a couple guys working there who are retired from their trades instead of being a former stockbroker assigned to the plumbing section,,, you could also try hydraulic cement & see if that works - you have the advantage of actually SEEING the problem !